Motor rigidity is a hallmark symptom of Parkinson's disease (PD), and there is increasing evidence for rigidity in non-motor domains such as cognition and personality. The question is raised as to whether such rigidity extends to visual perception, which is known to be abnormal in PD. Research on perception with healthy adults has focused on perceptual reversibility, which is the ability to switch the image of a bistable ambiguous figure. Perceptual reversibility depends on brain areas that are typically affected by PD, including corticostriatal-thalamocortical circuitry. The proposed study aims to examine perceptual rigidity in PD, the mechanisms associated with this perceptual phenomenon, and its association with motor, cognitive and personality rigidity. Non-demented PD patients and age-matched normal control participants will be presented with bistable (reversible) figures and will be assessed for their natural ability to switch from one perceptual interpretation to the other. Their alternation rate for binocular rivalry (perceptual dominance of one image over the other one) will be assessed. In order to explore possible mechanisms underlying rigidity, several experiments will be conducted including examinations of volitional control, visuospatial attention, and the ability to engage in bottom-up processing when stimulus features are degraded. The participants will be assessed for cognitive rigidity (neuropsychological tests of executive function including perseveration), motor rigidity (gait measures of relative phase between arm and leg movements and between pelvic and thoracic rotation), and personality rigidity (questionnaire). It is hypothesized that relative to the control group, individuals with PD will show reductions in perceptual reversibility in that they will have a reduced ability to intentionally "let go" of the dominant percept due to compromised fronto-striatal-parietal networks that support the cognitive abilities of selective attention and set-shifting. Second, it is hypothesized that bottom-up information will have a weaker effect on perceptual dominance in PD patients than in control participants. Third, it is hypothesized that in the PD group, there will be significant correlations between perceptual rigidity and cognitive, motor, and personality rigidity as these motor and non-motor PD symptoms all emanate from the direct and indirect influence of the dysfunctional basal ganglia. Understanding the nature of perceptual rigidity in PD can inform the development of perceptual interventions to enhance PD patients'quality of life and independence.